The present application relates to a functional molecular device employing a functional molecular element exhibiting its functions under the operation of an electrical field.
In the field of a functional molecular element, realizing its functions under the action of an electrical field, researches are now being conducted for employing the nano-technology, which is a technology of observing, fabricating and utilizing a miniaturized structure with the size of the order of one hundred-millionth of a meter (10−8=10 nm).
Towards the end of eighties, a microscope of ultra-high precision, called a scanning tunneling microscope, was invented, whereby it became possible to observe individual atoms or molecules. With the use of the scanning tunneling microscope, not only may the atoms or molecules be observed, but also these may be handled individually. For example, a report has been made of an instance of arraying atoms in the form of a letter or character on the surface of a crystal. However, even though the atoms or molecules may be handled in this manner, it would not be practical to handle an enormous number of atoms or molecules one by one to assemble a new material or device.
If the atoms, molecules or groups thereof are handled to form a nanometer-scale structure, a new enabling ultra-precision machining technology is necessitated. This nanometer-scale ultra-fine machining technology may roughly be classified into the following two systems.
One of these systems is the so-called top-down system in which a silicon wafer of a larger size is machined to a small size to the limit of machining and an integrated circuit is formed thereon. This system has so far been used for the fabrication of a variety of semiconductor devices. The other is the so-called bottom-up system in which atoms or molecules, as the units of the miniscule size, are used as components, and a target nanometer-scale structure is fabricated by assembling these small components together.
As for the limit towards reducing the size of a structure in accordance with the top-down system, there is a famous Moore's law propounded in 1965 by Gordon E. Moore who is a joint founder of the Intel Corporation. This law states that the degree of transistor integration is doubled in eighteen months. Since 1965, the industrial circles of semiconductors succeeded in raising the degree of transistor integration for over thirty years, as predicted in the Moore's law.
The road map ITRS (International Technology Roadmap for Semiconductor), for the fifteen years to come, as publicized by the United States Semiconductor Industry Association (SIA), European Semiconductor Industry Association, Japan Electronics and Information Technology Industries Association, Korean Semiconductor Industry Association and Taiwan Semiconductor Industry Association, expresses an opinion that the Moore's law will continue to remain valid.
The ITRS is composed of a short-span road map, valid until 2013, and a long-span road map, valid until 2020. The short-span road map states that, in 2013, the process rule for the semiconductor chip and the gate length of a microprocessor will become 32 nm and 13 nm, respectively. The long-span road map states in 2020, the process role for the semiconductor chip and the gate length of a microprocessor will become 14 and 6 nm.
The more the semiconductor chip is miniaturized, the higher becomes its operating speed and the lower becomes the power consumption. Moreover, the number of components that may be produced from a sole wafer becomes larger, with the production cost being correspondingly lowered. This accounts for competition among microprocessor manufacturers for miniaturizing the process rule and for raising the integration degree of transistors.
In November 1999, a laboratory group of United States has clarified the results of epoch-making researches in the miniaturization technology. These researches are for a method of designing a gate on an FET (field-effect transistor), termed FinFET, developed by a group of Professor Chenming Hu specializing in the computer science in Berkeley School of the University of California, USA. This method enables a number of transistors, about 400 times as many as that of the conventional technology, to be fabricated on the semiconductor chip.
The gate is an electrode controlling the flow of electrons in the channel in an FET. In the currently accepted routine designing, the gate is placed on the semiconductor surface in a parallel relation thereto for controlling the channel from one side. With this structure, an electron flow cannot be interrupted except if the gate is of a length exceeding a certain length. Hence, the gate length has so far been taken to be among the factors restricting the miniaturization of the transistors.
With the FinFET, on the other hand, the gate is of a forked configuration lying on both sides of the channel in order to effectively control the channel. It is possible with the FinFET to further reduce the size of the gate length and the transistor than with the conventional transistor structure.
The gate length of a prototype FET, prepared by the laboratory group, is 18 nm, which is one-tenth of the currently accepted ordinary gate length. This gate length of the laboratory group compares favorably with the size for the year 2014 specified in the long-span road map of ITRS. It is stated that a gate length equal to one-half the above gate length may be possible. Since Professor Hu et al. states that they are not willing to acquire patent for the technology in expectation of wide acceptance in the semiconductor circles, it may be predicted that FinFET will be in the mainstream of the fabrication technique in future.
However, it has also been pointed out that the Moore's law will reach a limit by the law of nature sooner or later.
For example, in the semiconductor technology, now in the mainstream, a semiconductor chip is fabricated as a circuit pattern is printed on a silicon wafer by a lithographic technique. For raising the degree of miniaturization, the resolution has to be raised. For raising the resolution, it is necessary to put the technique of utilizing light of a shorter wavelength to practical use. Since physical limitations are imposed on the wavelength of light that can be exploited in the lithographic technology, there is necessitated a breakthrough from a different aspect in order to surmount the limitations imposed on the wavelength.
There is also fear that the quantity of heat evolved per semiconductor chip is excessively increased with increase in the degree of integration, thus possibly causing malfunctions or thermal destruction of the semiconductor chip.
In addition, according to experts'predictions, if the semiconductor circles continue their endeavor in reducing the ship size, investment or process costs are increased and, in conjunction with the lowered yield, the semiconductor industry will become inviable in ca. 2020.
As a new technology for making a breakthrough of the technological barrier inherent in the top-down system, the researches directed to endowing individual molecules with the functions as electronic components are stirring up notice. The target of these researches is an electronic device, such as molecular switch, formed by an individual molecule, and which is prepared in accordance with the bottom-up system.
Researches for fabricating a nanometer scale structure of metals, ceramics or semiconductors in accordance with the bottom-up system are also underway. However, if molecules, which are separate and independent from one another and which, in case the difference in shape or functions is taken into account, are of millions of species, are ingenuously exploited, it may be possible to re-design and fabricate the devices (molecular devices), having features totally different from those of the conventional devices, in accordance with the bottom-up system.
The width of an electrically conductive molecule is as small as 0.5 nm. With a linear array of these molecules, it is possible to achieve a wire having a density thousands of times higher than one of a line of a width of the order of 100 nm realized with the current integrated circuit technology. On the other hand, if a sole molecule is used as a storage element, the recording density as high as about one thousand times that of the DVD (Digital Video Disc) may be achieved.
The molecular device is synthesized by a chemical process, in a manner different from the case of a conventional semiconductor silicon. In 1986, a world's first organic transistor, formed of polythiophene, was developed by Yusi Koezuka of Mitsubishi Electric Co. Ltd., Japan.
On the other hand, an organic electronic device could be prepared with success by a laboratory group of the Hewlett-Packard (HP) of USA and the Los Angeles school of the University of California, and the report thereon was made in ‘Science’ in July 1999. The contents of the studies by the laboratory group are concretely disclosed in U.S. Pat. Nos. 6,256,767B1 and 6,128,214. The laboratory group fabricated switches, using molecular films of several millions of organic rotaxane molecules, and joined these molecular switches together to form an AND gate as a fundamental logic circuit.
On the other hand, a joint laboratory group of University of Rice and University of Yale of USA succeeded in fabricating a molecular switch in which a molecular structure is changed by electron implantation under application of an electrical field in order to perform a switching operation. The contents of the researches by the laboratory group are disclosed in J. Chen, M. A. Reed, A. M. Rawlett and J. M. Tour, “Large on-off ratios and negative differential resistance in a molecular electronic device”, Science, 1999, Vol. 286, 1552-1551, and in J. Chen, M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin and J. M. Tour, “Conductance of a molecular junction”, Science, 1997, Vol. 278, 252-254. The function of repeated on-off has not been accomplished by the group of the HP Inc. and the Los Angeles School of University of California.
The professor J. Tour, University of Rice, who is specializing in chemistry, and who succeeded in the synthesis, states that the production cost of the molecular switch may be one several-thousandth of that of the conventional system because no expensive clean room, ordinarily used for semiconductor fabrication, is required, and that a hybrid computer of molecules and silicon will be fabricated in five to ten years to come.
In 1999, the Bell Laboratories (Lucent Technology Inc.) fabricated an organic thin-film transistor, using a pentacene single crystal. This transistor was of characteristics which favorably compare with those of an inorganic semiconductor.
Even though the researches in a molecular device, having the function of an electronic component, are going on briskly, the researches so far made were mostly directed to driving with light, heat, protons or ions (see “Molecular Switches”, WILEY-VCH, Weinheim, 200, edited by Ben L. Feringa), whilst only a limited number of the researches were directed to driving with an electrical field.
Meanwhile, the molecular elements, driven by the electrical field, so far proposed in the art, were only those exploiting the changes in the physical properties of the molecules themselves, caused under the influence by the electrical field. That is, the molecules themselves are thought of as single elements and the states of the electrons are varied by the electrical field. For example, in an organic FET, carrier migration in an organic molecule is modulated by changes in an electrical field acting on an organic molecule in a channel region.